Bicycle Mechanics - Fr Derailleur/Chainring--How low can I go?

I currently ride a 52/42 chainring set on my road bike. The crankset and front derailleur are old (ca. 1983) Suntour Superbe. I am thinking of replacing the 42 with a 39 (or lower), so that I can squeak out a higher cadence on the steeper climbs.
What are the limitations here? Can I comfortably go lower than 39 (say 38 or 37)? Provided I can get my hands on a chainring that fits the cranks, will the derailleur be able to handle it? I notice that there is a good 1.5 cm. of space (measured vertically) from the chain to the bottom of the derailleur cage.

Will such a combination (say, 52/39 or 52/37) make up-shifting or down-shifting problematic? (I know that 53/39 is a pretty standard combination these days, but that’s with newer equipment.)

By the way, the rear Mavic hubs are threaded (not cassette), so getting a new rear cluster is not really an option. I was lucky to find a threaded 14-24 (6 cogs). Apparently there is a 14-28 (7 cog) available, but I am not sure whether the 7 will fit.

As always, thanks in advance for any wisdom shared.

Cheers,
Jamie

Your small chainring size is dictated by BCD. You can go with a 38 on a 130 BCD and a 39 on a 135BCD.Not sure what yours is as suntour made both 130 and 135.My Superbee pro is 130 and the cyclones are 135.Your derailer will handle the 38-52/53 spread ok. Freewheels other than the Millard Heliomatic use a standard threaing.A 7 speed will typically work on a 6 speed hub,but sometimes may need a longer axel of thin spacer on the right side for dropout clearance.

If you want a low-ratio double you need a crank with a smaller Bolt Circle Diameter than Shimano road double. TA make a very nice expensive one, and Stronglight make a good enough cheap one.

Ive used a 36/48 on a Stronglight. Ive even seen them with a 28/38 double, for a woman who rides slowly around a town with steep hills. The Stronglight can be converted to a triple with some spacers and longer bolts. All 3 rings use the same bolts.

The front mech should handle a 36 OK, but it helps if you use a smaller big ring, so you can drop the mech a few mm.

You can still get hold of decent freewheels from 12-28 in the UK. Sources include SJScycles, Settle Cycles and Highpath Engineering. The latter shop will build custom setups for you and even custom manufacture chainrings.

http://www.argonet.co.uk/highpath/cycle_/faqs.htm#intro

Sugino makes a inexpensive 110BCD crank that allows a wide range of ring sizes.Try www.branfordbike.com The old school mtb cranks work well too if you can scrounge one.

Originally posted by pokey
Sugino makes a inexpensive 110BCD crank that allows a wide range of ring sizes.Try www.branfordbike.com The old school mtb cranks work well too if you can scrounge one.
110/74BCD is old-school MTB, and I second your recommendation. One can go down to 24T on the grannie chainring, and can get a perfectly adequate high gear with a 48/13, 46/12, or even a 44/12 combination. (I run 48-45-34 / 13-23 with a Shimano 600 front and SunTour short-cage cyclone rear.)

Thanks for all the tips. Budget-wise, I'll probably just go with a 39 on my current cranks.

By the way, Michael, I email the guys at Highpath. Unfortunately, they are no longer manufacturing freewheel cogs, so they have only a few sizes left in stock. But, thanks for the tip.

Cheers,
Jamie